Titanium alloys and fiber composites are the benchmark classes of materials for fan and compressor blades in commercial airline engines. One reason for the materials being so broadly adopted is that regulations require an engine in commercial service to be capable of ingesting birds while allowing for continued operation or safe and orderly shutdown of that engine. Another reason is that blades must resist cracking from nicks and dents caused by small debris such as sand and rain. Engines with titanium fan blades as well as certain reinforced fiber composite fan blades with adhesively bonded metallic leading edge sheaths are the most common blades used to meet these criteria.
While titanium blades are relatively strong, they are heavy and expensive to manufacture. Composite blades offer sufficient strength and a significant weight savings over titanium, but they are expensive to process. Further, due to their relatively low strain tolerance, composite blades require a greater thickness than otherwise equivalent metal blades to meet bird strike requirements. Greater blade thickness reduces fan efficiency and offsets a significant portion of weight savings from using composite materials.
Blades made of aluminum or aluminum alloy can result in significant weight savings. However, aluminum alloy blades are softer and lower in strength than past titanium or composite blades. Aluminum blades are particularly susceptible to corrosion and erosion, particularly when it is in contact with a dissimilar metal, such as a titanium sheath or fan hub. Corrosion or erosion can lead to crack initiation which could result in failure of the aluminum blade. Aluminum blades are also prone to pitting from foreign object damage and sand erosion. This pitting can impart a local stress concentration and reduced fatigue capability of the aluminum alloy. A leading edge sheath made of titanium or nickel can give the aluminum blade added protection without significantly increasing the weight.